The present relates to actively shielded superconductive magnetic resonance (MR) magnets which are cooled by refrigeration without the use of consumable cryogens.
Magnetic resonance imaging magnets used in medical diagnostics employ uniform high intensity DC magnetic fields in the interior of the magnet. A stray field surrounds the magnet and can adversely affect electronic equipment near the magnet unless the electronic equipment is individually shielded or the magnet stray field is reduced. Pacemakers, for example, have been found to be affected by stray fields as low as 10 gauss, (the earth's magnetic field is 0.5 gauss), so that uncontrolled access to the areas surrounding the magnet have to be restricted.
One approach to the stray field problem has been to locate the magnet in a separate building reserved exclusively for the purpose of diagnostic imaging. This can be costly since it depends on providing additional real estate. At times, such as in crowded urban areas, it is not feasible to provide the extra space.
Another approach is to surround the room where the magnet is located with ferromagnetic sheets which act to inhibit the extent of the stray field in the room. The disadvantages to this approach are that the shielding can add up to one hundred tons to the weight of the room, usually requiring structural modifications to the building. Also any dissymmetry in the ferromagnetic material surrounding the magnet can adversely affect the homogeneity in the working volume of the magnet leading to imaging distortions.
Still another approach to the stray field problem is described in U.S. Pat. No. 4,646,045 in which the magnet is surrounded by enough ferromagnetic material to suppress the stray field. This approach has several potential disadvantages. The shield is designed for optimum operation at only one field level in the magnet and the proximity of the ferromagnetic material to the magnet makes the homogeneity of the working volume susceptible to any dissymmetry introduced during the manufacturing process.
Yet another approach is to use an actively shielded magnet, described in U.S. Pat. No. 4,587,504, issued May 6, 1986 to Brown, et al. Brown et al describes creating two homogeneous fields of different strengths using two different sets of coils, with the fields subtracting from one another thereby producing a uniform magnetic field. Active shielding in conventionally helium cooled MR magnets is not as attractive or cost effective as passive or room shielding for 1.5 Tesla (1 Tesla=10,000 gauss) magnets since it increases the cryostat outer diameter by 50%, close to 3 meters, and doubles the magnet ampere-turns.
It is an object of the present invention to provide a low cost, lightweight actively shielded MR magnet without cryogens with about the same bore and outside diameter as a conventional helium cooled magnet of the same field strength and homogeneity.